1. Field of the Invention:
The present invention relates to an electronic component, such as a capacitor, and a method for manufacturing the same.
2. Description of the Prior Art:
A capacitor as an electronic component has the structure of a dielectric interposed between a pair of electrodes. As the dielectric, aluminum oxide, titanium oxide, tantalum oxide, or a plastic film is generally used. An aluminum capacitor, for example, is manufactured by the following method. First, an aluminum oxide layer is formed as a dielectric on an aluminum surface by electrochemical oxidation, then a manganese dioxide layer is formed on the aluminum oxide layer by thermal decomposition, and finally the manganese dioxide layer is coated with conductive carbon. Thus, the above aluminum capacitor has a structure of the aluminum oxide layer and the manganese dioxide layer as dielectrics interposed between the aluminum layer and the conductive carbon layer as electrodes.
Other methods for forming a conductive electrode on a dielectric than the above coating of conductive carbon have also been proposed, such as evaporation of a conductive material and electroless plating in which a conductive material is directly deposited on a dielectric.
In the electroless plating, however, it is difficult to deposit a conductive material on the manganese dioxide layer which is highly oxidative. To avoid this trouble, the conductive material may be deposited directly on the aluminum oxide layer. The aluminum oxide layer, however, does not allow the conductive material to be formed uniformly thereon. As a result, a capacitor thus obtained is not provided with stable characteristics, especially in leakage.
Therefore, an electrode of a capacitor is generally formed by coating conductive carbon or by evaporating conductive metal. However, a capacitor using conductive carbon as an electrode has an increased inductance at radio frequency, which makes it difficult to obtain an ideal performance of the capacitor. In recent years, a capacitor having a polypyrrole layer which is a conductive polymer formed between a dielectric and a conductive carbon layer has been developed in order to prevent the inductance increase at radio frequency and thereby to realize the coverage of a wider frequency range including radio frequency.
The above-mentioned capacitor having the polypyrrole layer may or may not have a manganese dioxide layer on the dielectric layer. In either case, the resultant capacitor shows excellent characteristics in the radio frequency range. The existence of a manganese dioxide layer as another dielectric advantageously compensates the insulation in the event of incomplete insulation by the dielectric aluminum oxide. This is further advantageous in that before being electrochemically polymerized the polypyrrole layer can be chemically oxidized due to the high oxidation property of manganese dioxide, allowing the conductive polypyrrole layer to attach further firmly to the dielectric. Thus, the formation of a manganese dioxide layer is important for high efficiency.
As mentioned above, the capacitor using polypyrrole as a conductive polymer can cover the radio frequency range, which is also possible in a ceramic capacitor, and has good mass-productivity. However, the heat resistance of the capacitor having the polymer electrode is as low as about 120.degree. C. Therefore, enhancing the heat resistance is required in order to ensure high reliability.
Furthermore, in the recent advancement of electronic apparatuses, a variety of shapes, especially fine shapes of capacitors have been required. A manufacturing process of a fine-shaped capacitor generally requires a photolithograph step where a fine pattern is formed using a photoresist. In general, a fine-shaped capacitor is manufactured by the following process. First, the surface of a conductive substrate having an insulation formed thereon is coated with a photoresist, exposed to light through a mask having a desired pattern, and then subjected to development and etching, to obtain a conductive substrate having a finely shaped insulation thereon. An electrode is then formed corresponding to the shape of the insulation, thus to manufacture a fine-shaped capacitor.
In the photolithograph step for fine shaping of a capacitor, an organic photoresist is generally used. In this case, a large quantity of volatile organic solvents having a low boiling point, such as acetone, isopropyl alcohol, toluene, xylene, and Freon (trademark), are used when the resist is applied or removed. Therefore, careful control for exhaust gas is required in the steps using these solvents. In particular, in recent years, the atmospheric pollution by exhaust gas, especially by Freon (trademark) has given rise a problem on a global scale.
In a ceramic capacitor, a microwave element, and the like using ceramic as a dielectric, electroless plating is generally used for forming an electrode, in which metal such as copper and nickel is deposited on the ceramic surface to form an electrode.
A conventional method for forming an electrode on barium titanate, one of typical ceramic materials, is described below.
First, a ceramic substrate mainly composed of barium titanate is degreased with an alkaline solution. After a complete cleaning off of the alkaline solution, the surface of the ceramic substrate is roughened by etching with hydrofluoric acid. Next, after a complete cleaning off of the etchant, two catalyst-providing steps, that is, sensitizing and activating, are performed. Then, the ceramic substrate is washed with water and then subjected to electroless plating so as to deposit metal on the ceramic surface for forming an electrode. The first catalyst-providing step of sensitizing uses a tin chloride/hydrochloric acid solution, and the second catalyst-providing step of activating uses a palladium chloride/hydrochloric acid solution. In the latter activating step, it is considered that a bivalent tin ion existing on the ceramic surface is oxidized into a tetravalent tin ion, while at the same time a bivalent palladium ion is reduced into palladium metal which attaches to the surface.
The deposited conditions of metal such as copper and nickel on the ceramic surface having palladium metal deposited thereon, especially adhesion strength of the metal to the ceramic surface, greatly depends on whether the tin chloride has been tightly attached to the ceramic surface. The above roughening of the ceramic surface is made to increase the adhesion strength. This roughening also contributes to attaining uniform electrical characteristics of a capacitor or a microwave element manufactured.
As described above, degreasing and roughening of a ceramic surface are required for forming an electrode thereon. However, excessive roughening will weaken the ceramic itself. Further, the treatment liquid may be left inside the cavities of the excessively roughened surface, causing it to lessen the adhesion strength of the deposited metal to the surface. The remaining liquid in the cavities may also corrode the deposited metal. Moreover, the hydrofluoric acid used for etching is hazardous, requiring careful handling. For the above reasons, a technique which can eliminate the roughening and which can provide high adhesion strength of the electrode to the ceramic surface and stabilize the electrical characteristics is required.